Every application filed with respect to insurance of mortgages on a two-, three-, or four-family dwelling, or a single-family dwelling which is one of a group of 5 or more single-family dwellings held by the same mortgagor, must be accompanied by a contract in form satisfactory to the Commissioner, signed by the proposed mortgagor covenanting and agreeing that so long as the proposed mortgage is insured by the Commissioner the mortgagor will not rent the housing or any part thereof covered by the mortgage for transient or hotel purposes, together with the mortgagor's certification under oath that the housing or any part thereof covered by the proposed mortgage will not be rented for transient or hotel purposes. For the purpose of this subchapter rental for transient or hotel purposes shall mean (a) rental for any period less than 30 days or (b) any rental if the occupants of the housing accommodations are provided customary hotel services such as room service for food and beverages, maid service, furnishing and laundering of linen, and bellboy service.